1. Field of the Invention
This invention relates to charged particle beam projection systems and more particularly to the imaging sections of an electron or ion beam column.
2. Description of Related Art
In a particle beam projection lithography system with sufficient beam current to provide useful tool throughput there is a problem that the Coulomb interaction causes a shift in the position of the image plane and a change in nominal magnification which vary depending on the transmitted beam current, i.e., exposure pattern density.
For a hypothetical electron beam column under consideration, Monte Carlo calculations of the effect suggest image shifts of approximately 50 microns and magnification changes of approximately 0.1%, for a 20 microAmpere change in transmitted beam current. Tolerable image shifts and magnification changes are about 10-100 times smaller than these values.
Dynamic refocusing to correct shifts of the image plane of an electron beam system in response to beam current changes is known from shaped beam lithography systems, and character projection systems, but to our knowledge, compensation of magnification effects has never before been addressed.